Normal radiography is performed using essentially a point source of X-rays whose beams expand, pass through the tissue being investigated and expose an X-ray sensitive emulsion in accordance with the attenuated intensity of the X-rays. Since the X-rays are not projected in parallel beams, exact size determinations of the tissue are difficult from observation of the exposed emulsion. This is because the angled beams cause magnification which is dependent upon the distances between the point source, the portions of the patient of interest, and the emulsion.
Devices such as the devices shown in U.S. Pat. No. 4,048,507 can be used to determine the magnification at predetermined locations between an X-ray source and the emulsion film. However, for these determinations to provide accurate results in certain diagnostic procedures, the exact relative location of the organ being investigated also must be known. Since patients' physiologies vary greatly, especially when disease or abnormalities are involved, there has been a need to provide a scaling device which can be located centrally within the organ of interest so that the magnification factor can be determined.
The inability to determine magnification is a particularly acute problem in angiographic estimation of left ventricular volume of a patient's heart. Most methods assume the left ventricular chamber to be ellipsoidal in shape and the following is a typical emperical equation for determining the volume: ##EQU1## where A=planimetered area of ventricular cavity silhouette, and L=the longest length of ventricular cavity silhouette.
The small standard errors observed in volume estimates derived from angiograms indicate that the geometry, orientation, and shape of the left ventricular cavity in the thorax are mathematically predictable with a high degree of accuracy. Conversely, the extent to which the estimates of volume may be effected by changing the orientation of the left ventricular cavity in relation to the radiographic filming plane shows that the accuracy of any angiographic estimate of volume depends on a normal orientation of the left ventricular cavity in the chest and the knowing of its location, particular when single plane films are used.
Magnification of the cavity silhouette results because the non-parallel X-ray beams originate from a virtual point source and because short filming distances are used in cardiac angiography. The degree of magnification can be determined by knowing the height of the left ventricular chamber from the X-ray tube focal spot (h.sub.1), the distance from the focal spot to the film or in the case of cinefluoragraphy, the input phosphor of the image intensifier (h) and the measured image semi-axes (a.sub.1). The true semi-axes (a) can be calculated using the parallax method or similar sides of similar triangles hypothesis as shown. EQU a=a.sub.1 h/h.sub.1 Equation ( 2)
The location of the level of the left ventricular cavity is difficult to determine while the patient is in the position of filming because the whole procedure is performed either with the X-ray source and X-ray detector at an angle to vertical or with the patient at an angle to vertical. Heretofore the common method has been to use a one centimeter square grid filmed after the patient has been filmed and placed at what is the estimated to be the level of the center of the patient's left ventricular cavity. The method has two advantages. First it allows the film to be projected for the purposes of measurement at any field size eliminating one potential source of error, and secondly the degree of image magnification can be determined from the ratio of the known or true area of a block of squares in the grid and the area of this block drawn from the projected film and measured with a compensating polar planimeter. However, to be useful the grid must be located at the right height and this is almost impossible to determine accurately especially if abnormal physiology is involved.
Some investigators have suggested the magnification correction factors can be derived from measurement of the diameter of the cardiac catheter used in the angiogram knowing its true external diameter. This has proved to be a source of large error in the calculation of the volume because the X-ray beam does not originate from a true point source but from a focal spot of finite width. X-rays from both sides of the focal spot demarcate the edges of the catheter so that the margin of the X-ray image exhibits a double edge or penumbera. The indistinct margin which results from the penumbera effect is particularly apparent in objects of smaller dimensions such as the catheter's width and can result in an error of approximately 50% in the volume estimation at conventional filming distances.
Therefore, there has been a need to provide means for calibrating the magnification factor of X-ray studies particularly during angiographic estimation of left ventricular volume which has been unsolved heretofore.